Membrane core for solid propellant rocket motors and casting method using same



July 19, 1966 P. s. CRAIG 3,261,891

MEMBRANE GORE FOR SOLID PROPELLANT ROCKET MOTORS AND CASTING METHOD USING SAME Filed March 20, 1964 4 Sheets-Sheet 1 Preston 6. Craag INVENTOR.

BYMW

ATTORNEY July 19, 1966 P. s. CRAIG 3,261,891

MEMBRANE CORE FOR SOLID PROPELLANT ROCKET MOTORS AND CASTING METHOD USING SAME Filed March 20, 1964 4 Sheets-Sheet 2 Preston 8. Craig INVENTOR.

BY M

AT TORNE Y July 19, 1966 P. s. CRAIG 3,251,891

MEMBRANE CORE FUR SOLID PROPELLANT ROCKET MOTORS AND CASTING METHOD USING SAME Filed March 20, 1964 4 Sheets-Sheet 5 m I? W I a i 22 [K a Presfon 5. Craig INVENTOR.

ATTORNEY July 19, 1966 P. s. CRAIG 3,261,891

MEMBRANE CORE FOR SOLID PROPELLANT ROCKET MOTORS AND CASTING METHOD USING SAME Filed March 20, 1964 4 Sheets-Sheet 4 I J Preston 8. Craig INVENTOR BY MW AT TORNE Y United States Patent Filed Mar. 20, 1964, Ser. No. 353,450 Claims. 01. 264-3) This invention relates to improvements in membrane cores that are especially adapted for use in forming the cavity configuration in solid propellants as they are cast into motor cases for solid propellant rocket motors.

The burning time in solid propellant rockets demands central, internal cavity configurations so that the burning of the solid propellant progresses from the cavity configuration outwardly to to the liner covering the inner surface of the motor case. Thus the solid propellant for most of the burning time of the solid propellant functions as an insulation for the motor case. I

The cavity configuration is, therefore, an important feature of the solid propellant rocket motor; and it must be so formed that the internal burning of the solid propellant begins with the surface of the solid propellant defining the cavity configuration and progresses radially toward the inner surface of the wall of the motor case. The cavity configuration normally extends the entire length of the motor case centrally of the solid propellant cast therein and is customarily formed by casting the solid propellant around a mandrel or core which is withdrawn after the solid propellant has been properly cured.

Before the size of the motor case became a major factor, the mandrels or cores had been rigid, being made of steel, aluminum, plastic and foam or' a combination of the latter materials. Hollow mandrels or cores have been substituted for the solid mandrels or cores, but such were still of a rigid nature and somewhat limited to size v and application.

Some of the disadvantages inherent with mandrels or cores of this type were the extremely large lifting force that was required to remove such cores from the solid propellant after it had cured and, if the release agent that was applied to the surface of the mandrel or core did not function properly, the solid propellant would be permauently damaged and would have to be removed and a new solid propellant cast in the motor case. Such a procedure would be time-delaying and very costly and provided a good reason for developing a mandrel or core which would not have this disadvantage.

Another requirment for the use of such mandrels or cores was the large amount of protected headroom that was necessary for the insertion and removal of the mandrel or core from the motor case.

Segmented mandrels and cores were used; and while such cores could be removed from the solid propellant with less lifting force, such cores were expensive to manufacture because of the precision fit that was necessary between the segments to prevent the solid propellant from leaking between the segments and creating a safety hazard.

The use of such cores, therefore, greatly increased the cost of the solid propellant rocket motors, and it became 3,261,891 Patented July 19, 1966 provide a mandrel or core that was formed from a flexible membrane material that could be filled with a filler whose density in cross section would balance the crosssectional density of the solid propellant on the exterior surface of the mandrelor core.

With such a mandrel or core forming the subject of the instant invention, no removal machinery would be required. The filler could be drained or pumped from the motor case; and the membrane material could be stripped from the surface of the cavity configuration, and only a moderate stripping force would be required. However, if the membrane material was of a consumable material, the solid propellant could be fired without removing the membrane material.

Since only a small amount of headroom for the removal of the mandrel would be. required, it could be performed when the motor case waseither in a vertical or horizontal position.

The cost of such a mandrel or core would be inexpensive as compared with rigid cores; and since the membrane material would be leak-proof, it would require no elaborate arrangement of sealing means and the filler could be of an inexpensive type that could be disposed of or reused if necessary.

Another object of the invention, therefore, was to provide a membrane mandrel or core that was relatively simple in construction and operation, inexpensive to manufacture and changes in the mandrel or core geometry could be accomplished with only a small additional cost and a minimum amount of time.

A further object of the invention is to provide amembrane mandrel or core that will accommodate any reduction in the volume of the solid propellant that may be occasioned by the shrinkage thereof for the reason that the membrane mandrel or core will remain in contact with the solid propellant at all times and be in hydro-static balance therewith at all times.

A still further object of the invention is to provide a membrane mandrel or core that can be retained in the cavity configuration at all times and thus prevent the slump or distortion of the solid propellant.

A still further object of this invention is to provide a.

membrane mandrel or core that will be simultaneously in contact with the solid propellant and the filler so that the heat transferrance between all three will be high.

Thus the filler can be appropriately heated or cooled and thus be a factor in controlling an even cure of the solid propellant and create, if necessary, a rapid cooldown of the solid propellant with a minimum induced strain thereof.

With the above and other objects and advantages in View, the invention consists of the novel details of construction, arrangment and combination of parts more fully herein-after described, claimed and illustrated in the accompanying drawings in which:

FIGURE 1 is an exploded, perspective view of the membrane mandrel or core embodying the invention.

FIGURE 2 is a schematic view showing the membrane core in position within a motor case and with the solid propellant starting to be cast therein.

FIGURE 3 is a similar View with the solid propellant at approximately its halfway level, and

FIGURE 4 is a similar view showing the solid propell-ant completely cast into the motor case.

Referring more in detail to the drawings wherein hke parts are designated by like reference numerals, the reference numeral 10 is used to generally designate a membrane mandrel or core assembly embodying the invention.

The membrane mandrel or core assembly 10 comprises a head end anchoring member 11 which is not to be lim-- ited to the configuration which is shownbut will, re-

i) gardless of its configuration, conform to the cavity con figuration that is to be created in a solid propellant 12 as it is cast into a motor case 13.

A centering bar or rod 14 extends through the motor case 13 to project outwardly through an opening 15 in the head end 16 of the motor case 13. The centering bar or rod 14 also extends through a central opening 17 in the anchoring member 11 and is fixed rigidly thereto so that as the centering bar or rod 14 is withdrawn from the motor case 13 the anchoring member 11 will also be withdrawn from the motor case 13.

Slidably mounted on the centering bar or rod 14 by means of a central opening 18 therein is a follower 19. The follower 19 is hollow and is also of the same configuration as the cavity configuration in the solid propellant 12 and the anchoring member 11.

' Fixed to the centering bar or rod 14 adjacent the aft end 20 of the motor case 13 is an aft end centering member 21; and the centering bar or rod 14 extends through and outwardly thereof by means of a central opening 22. The aft end anchoring member 21 is also of the same configuration as the cavity configuration of the solid propellant 12, the head end anchoring member 11 and the follower 19.

Lifting cables 23 and 24 extend through openings 25 and 26 in the aft end anchoring member 21 to be secured at their terminals to the follower 19 at 27 and 28; thus the cables 23 and 24 can control the travel or sliding action of the follower 19 on the centering bar or rod 14 as the propellant 12 is cast into the motor case 13.

An inlet pipe or conduit 29 for the input of a filler 30 extends through an opening 31 in the aft end anchoring member 21 to terminate at its outlet at 32 in the follower 19. An outlet pipe or conduit 33 also extends through an opening 34 in the aft end anchoring member 21 to terminate at its inlet at 35 with the core cavity forward of the follower 19. The follower 19 is hollow in order that the level of the filler fluid 30 can be readily observed or measured. Thus the level which produces the necessary hydro-static balance between fluid and propellant can be maintained at all times.

In operation, a flexible membrane material 36 in a substantially tubular formation is secured at one end to the head end anchoring member 11; and the membrane mandrel or core assembly 10 is inserted into the motor case 13. When the membrane mandrel or core assembly 10 is in position, the opposite end of the flexible membrane material is secured to the aft end anchoring member 21. Suitable means 37 for supporting the aft end anchoring member 21 will be positioned on the aft end of the motor case 13; and such means may be adjustable so that the centering bar or rod 14 will coincide with the longitudinal axis of the motor case 13. The means 37 that is used for supporting the aft end anchoring member 21 will be capable of vertical movement in alignment with the longitudinal axis of the motor case 13 so that tension is placed on the flexible membrane material 36 causing it to conform to the configuration of the head end anchoring member 11, the follower 19 and the aft end anchoring member 21.

After the tension of the flexible membrane material 36 has been completed, the follower 19 is lowered by means of the cables 23 and 24 until it rests in nesting relation on the head end anchoring means 11.

The relation of the membrane mandrel and core assembly 10 at the start of the operation is clearly shown in FIGURE 1. The solid propellant 12 is then started to be cast into the motor case 13 in any well-known manner. The filler 30 in fluid form is also started to be fed .into the follower 19 by means of the inlet pipe or conduit .29. As the level of the solid propellant rises, the follower 19 is caused to rise by means of the cables 23 and 24. At the same time, the fluid filler 30 is being pumped into the space in the flexible membrane 36 vacated by the follower 19. Since the fluid filler 30 will possess the same cross-sectional density as the solid propellant 12, a hydro-static balance will be maintained across the flexible membrane material 36 so that it will not collapse but be retained under tension in the same configuration as the head end anchoring member 11, the follower 19 and the aft end anchoring member 21.

As the solid propellant 12 is cast into the motor case 13, the follower 19 will be maintained at the same level as is consistent with the rise of the solid propellant 12, and the fluid filler 30 will be maintained under sufficient quantity and pressure to maintain the flexible membrane material in the proper configuration. FIGURES 2 and 3 show the movement of the follower 19 as it leaves its contact with the head end anchoring means 11 and until it engages the aft end anchoring means 21 and the solid propellant 12 has been completely cast into the motor case 13.

When the casting has been completed, the inlet and outlet pipes or conduits 29 and 33 will be sealed off and the solid propellant 12 will be allowed to properly cure. The inlet and outlet pipes may also be used in conjunction with suitable external pumps and other equipment (not shown) to heat or cool and circulate the filler fluid within the propellant cavity to facilitate the cure and cooldown of the propellant or the temperature control thereof. This capability is an inherent advantage of the instant invention.

If it is desired, the membrane mandrel or core assembly 10 can be retained in the motor case until the motor case has been shipped to the firing pad or prepared for static testing. At this time, the fluid filler 30 may be pumped out of the motor case by means of the inlet and outlet pipes or conduits 29 and 33. The flexible membrane material 36 may then be stripped out of the cavity configuration by lifting the membrane mandrel and core assembly 10 out of the motor case, or it may be left in contact with the surface of the cavity configuration if it is of a combustible material.

There are many materials that may be used for the flexible membrane 36, but successful castings have been made with sheets of material that include synthetic rubbers that have or eliminate the use of longitudinal reinforcement therein. Composite laminated sheets of plastic containing a well-known oxidizer for aiding in the ignition or combustion of the flexible membrane have been used also. The fluid filler 30 can be one of many or a combination of well-known materials, However, successful results have been obtained from aqueous solutions of inorganic salts, liquid polymers and aqueous suspensions of inert solids.

It will be apparent from the foregoing description that a membrane mandrel or core assembly for use in casting solid propellant has been achieved; and it is believed that the construction and manner of use of the instant invention will be clear to those skilled in the art, it also being understood that variations in the manner of construction and materials used may be resorted to provided such variations fall within the spirit of the invention and the scope of the appended claims.

Having thus described the invention, what is claimed as new and desired to be secured by letters Patent is:

1. A membrane core assembly for use in forming the cavity configuration in the solid propellant cast into the motor case of a case-bonded solid propellant rocket motor comprising a tubular flexible membrane positioned in the motor case and parallel to the longitudinal axis thereof, anchoring means secured to both ends of said tubular membrane, centering means extending through said anchoring means and corresponding with the longitudinal axis of said motor case, a follower positioned on said centering means intermediate of said anchoring means, means connected to said follower for providing reciprocal movement thereof with relation to said anchoring means, and means for introducing a fluid into said tubular membrane to expand and retain said tubular membrane in expanded condition during the casting of the solid propellant in said motor case.

2. A membrane core assembly, as in claim 1, wherein said anchoring members and said follower are of similar configuration to the cavity configuration desired to be maintained in said solid propellant.

3. A membrane core assembly, as in claim 2, wherein means is provided to tension said tubular membrane so it will conform to the configuration of said anchoring means and said follower.

4. A membrane core assembly comprising a tubular flexible membrane, anchoring means secured to the opposite ends of said membrane, centering means extending through said anchoring means, means for injecting a fluid filler into said membrane and a follower mounted for reciprocal movement on said centering means and adapted to shape said tubular membrane to conform to the shape of said anchoring means during the injection of the fluid filler into said membrane.

5. A method for forming the cavity configuration in a solid propellant being cast into a motor case for solid propellant rocket motor comprising the steps of inserting a tubular membrane into the motor case, providing a reinforcing means for the membrane, tensioning the membrane to cause the membrane to conform to the shape of the cavity configuration, simultaneously injecting solid propellant into the motor case and a fluid filler into the tubular membrane and maintaining the level of the solid propellant and the fluid filler constant to cause the membrane to retain its preformed configuration during the casting of the solid propellant into the motor case.

References Cited by the Examiner UNITED STATES PATENTS 6/1964 Zinn 264-3 X 6/ 1965 Grace 2643 

4. A MEMBRANE CORE ASSEMBLY COMPRISING A TUBULAR FLEXIBLE MEMBRANE, ANCHORING MEANS SECURED TO THE OPPOSITE ENDS OF SAID MEMBRANE, CENTERING MEANS EXTENDING THROUGH SAID ANCHORING MEANS, MEANS FOR INJECTING A FLUID FILLER INTO SAID MEMBRANE AND A FOLLOWER MOUNTED FOR RECIPROCAL MOVEMENT ON SAID CENTERING MEANS AND ADAPTED TO SHAPE SAID TUBULAR MEMBRANE TO CONFORM TO THE SHAPE OF SAID ANCHORING MEANS DURING THE INJECTION OF THE FLUID FILLER INTO SAID MEMBRANE.
 5. A METHOD FOR FORMING THE CAVITY CONFIGURATION IN A SOLID PROPELLANT BEING CAST INTO AMOTOR CASE FOR SOLID PROPELLANT ROCKET MOTOR COMPRISING THE STEPS OF INSERTING A TUBULAR MEMBRANE INTO THE MOTOR CASE, PROVIDING A REINFORCING MEANS FOR THE MEMBRANE, TENSIONING THE MEM- 